In a conventional cellular system, antenna arrays of a network side device are usually arranged horizontally. Beams from a transmitter of the network side device may merely be adjusted in a horizontal direction, and are at a fixed down-tilt angle to each user in a vertical direction. Hence, various beamforming/pre-coding technologies may be performed on the basis of channel information in the horizontal direction. However, actually, a radio signal is transmitted in a space in a three-dimensional manner, so it is impossible to provide an optimal system performance through the fixed down-tilt angle. The beam adjustment in the vertical direction may play a very important role in enhancing the system performance.
Along with the development of the antenna technology, an active antenna capable of controlling each element independently has currently emerged, as shown in FIGS. 1A and 1B. FIG. 1A shows an active antenna with four ports. In FIG. 1B, a vertical dimension of a base station is segmented into three sectors, i.e., sector 1, sector 2 and sector 3, which may provide services to respective terminals therein. Each sector is allocated with a set of Channel State Information-Reference Signal (CSI-RS) resources, including several CSI-RS ports, e.g., four CSI-RS ports. Through this kind of antenna array, it is possible to dynamically adjust the beams in the vertical direction. For a Frequency Division Duplexing (FDD) or Time Division Duplexing (TDD) system, the three-dimensional beamforming/pre-coding operation needs to be performed on the basis of CSI fed back from a User Equipment (UE).
In a conventional CSI feedback approach, usually, one CSI-RS resource is configured for the network side device at each of the horizontal dimension and the vertical dimension, and each CSI-RS resource is transmitted from a set of antennae to the UE. Next, the UE measures the CSI-RS resource at the vertical dimension and generates the corresponding CSI, and measures the CSI-RS resource at the horizontal dimension and generating the corresponding CSI. Then, the network side device acquires information about downlink three-dimensional Multiple Input Multiple Output (3D-MIMO) beamforming based on the vertical-dimension CSI and the horizontal-dimension CSI from the UE.
In the above-mentioned feedback approach, the UE needs to feed back two pieces of CSI, each including a Rank Indicator (RI), a Pre-coding Matrix Indicator (PMI) and a Channel Quality Indicator (CQI), so there is a relatively large uplink feedback overhead.